Vertical couplers can be used to couple light from planar waveguides to optical fibers. Many vertical fiber-chip coupler designs have been proposed and demonstrated in the last decade. These couplers are based on grid coupling directly between the fiber and the on-chip waveguide. Therefore, this grid must be about as big as the fiber mode (e.g., about 10-30 μm) to support a similar field distribution and to maximize the coupling efficiency. This size constraint limits the coupler bandwidth to a full-width half-maximum (FWHM) of about 60 nm, which is on the same order as the fabrication errors for the center wavelength. It also limits the vertical couplers' usefulness for other applications, including optical phased array (OPAs).
Short antennas have been used to vertically couple radiation at radio frequency (RF) wavelengths. But these RF antennas do not operate on dielectric waveguides. Rather, RF antennas are fed from thin transmission lines, enabling the excitation of the antenna to occur at a single point or multiple points. Unfortunately, the dielectric waveguides used at optical wavelengths cannot be separated from the guided wave, so the guided wave interacts with the entire antenna instead of exciting a single point.